(1) Field of the Invention
The present invention relates to an active matrix substrate, a display panel using the substrate, and a testing method for the active matrix substrate and the display panel, and particularly relates to a protection circuit for the active matrix substrate and the display panel using the active matrix substrate.
(2) Description of the Related Art
Along with the increased demand for better display apparatus in recent years, Liquid Crystal Display (LCD) panels and organic electroluminescence (EL) display panels have been attracting attention as thin display panels with low power consumption. The display panels have two-dimensionally arranged pixels.
For example, in an active matrix organic EL display panel, thin film transistors (TFT) are provided at intersections of scanning lines and data lines. Storage capacitors, gates of driving transistors, and compensating circuits are connected to the TFTs. The TFTs are turned ON through selected scanning lines, input data signals and others are provided from the data lines to the driving transistors, the storage capacitors, and the compensating circuits. The driving transistors, storage capacitors, and the compensating circuits control brightness and timing of light emitted from the organic EL element. This configuration of the pixel driving circuit allows an active matrix organic EL display panel to have the organic EL element emit light until next scanning (selection). Consequently, even when a duty cycle increases, it does not result in decrease in display brightness.
However, the active matrix organic EL display panel has a complex pixel driving circuit configuration. Because of this, electric malfunctions such as uneven characteristics of the pixel driving circuit devices, short-circuited lines, and opened lines. In order to compensate the uneven characteristics and to improve yield at the time of manufacturing, it is necessary to perform sufficient testing on arrays of active matrix substrate during the manufacturing process, and testing on display operations of display panels after completion before mounting components to be mounted. At the time of testing, when an external measuring device and a measuring subject, that is, an active matrix substrate or a display panel are connected, or when no external measuring device is connected during the manufacturing process, there is a possibility that static electricity flows into an active matrix substrates, causing electrostatic breakdown of the pixel circuits. Patent Literature 1 (Japanese Unexamined Patent Application Publication No. H11-142888 discloses a configuration for protecting the display panel from the electrostatic breakdown at the time of manufacturing and testing an LCD.
FIG. 7 is a planar view schematically illustrating signal input terminals of the LCD apparatus according to Patent Literature 1. FIG. 7 shows an array substrate 501 on which display switching element such as a TFT (not illustrated) is bonded with a facing substrate 502. The facing substrate 502 includes a display region 503. In the array substrate 501, source lines 504 and gate lines 505 are provided as many as necessary for displaying images. Each of the source lines 504 has a testing signal voltage input terminal 506, and each of the gate lines 505 has a testing signal voltage input terminal 507. A short-circuit ring 508 made of low-resistance material such as metal is provided at the outer periphery of the array substrate 501. The short-circuit ring 508, the source lines 504, and the gate lines 505 are electrically connected via depletion-mode TFTs 509. The gate electrodes of the depletion-mode TFTs 509 are connected to the lines 510 on an arbitrary unit bases, and controlled by a gate voltage input terminal 511 on the unit basis.
With the configuration described above, during the manufacturing process of the LCD apparatus, no voltage is applied to the gate electrode of the depletion-mode TFTs 509, and all of the source lines 504, all of the gate lines 505, and the short-circuit ring 508 conduct via the depletion-mode TFT 509 which is in ON state. With this, it is possible to take measures for preventing electrostatic breakdown. When testing the LCD apparatus, for example, in the case of array testing probing array substrate for array defect, or in the case of panel lighting-up display testing probing the LCD panel before mounting components, it is necessary that probes and others contact the testing signal voltage input terminals 506 and 507 to apply individual testing signals and lighting-up signal voltages. In this case, it is necessary for all of the source lines 504 and the gate lines 505 to be electrically open to each other and with the short-circuit ring 508. Thus, negative OFF voltage is applied to the gate electrode of the depletion-mode TFT 509 from the gate voltage input terminal 511. With this, the lines are completely open, allowing highly precise testing and lighting-up display without being affected by signals from neighboring lines. When the depletion-mode TFTs 509 as ESD protecting elements are not necessary after the testing, the connection may be cut off at a cutting line 512. Alternatively, when leaving the depletion-mode TFTs 509 which is a ESD protecting element as a measure for preventing electrostatic breakdown during mounting process or after the manufacturing is complete, applying a negative OFF voltage to the gate voltage input terminal 511 at all times allows the lines to be open.